Semiconductor light emitting device

ABSTRACT

According to one embodiment, a semiconductor light emitting device includes: a first semiconductor layer of a first conductivity type; a second semiconductor layer of a second conductivity type; a light emitting layer; a conductive metal layer; and a first stress application layer. The first semiconductor layer contains a nitride semiconductor crystal and receives tensile stress in a (0001) plane. The second semiconductor layer contains a nitride semiconductor crystal. The light emitting layer has an average lattice constant larger than a lattice constant of the first semiconductor layer. The conductive metal layer has a thermal expansion coefficient larger than a thermal expansion coefficient of a nitride semiconductor crystal. The first stress application layer is provided between the second semiconductor layer and the light emitting layer. The first stress application layer relaxes tensile stress applied from the metal layer to the second semiconductor layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2012-256631, filed on Nov. 22,2012; the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a semiconductor lightemitting device.

BACKGROUND

Nitride semiconductors are used in semiconductor light emitting devices,and high performance devices are being put to practical use.

However, when a semiconductor light emitting device is formed byepitaxial growth of a nitride semiconductor crystal on a siliconsubstrate, which is less expensive and more efficient in manufacturingprocesses than a sapphire substrate, cracks, defects, etc. may begenerated due to the tensile stress included in the epitaxial crystallayer.

When device operation at high current density is required, thetemperature of the device becomes high, and cracks, defects, etc. due tothe stress caused by thermal expansion may be generated. Such cracks anddefects may degrade the device characteristics and in some cases maycause the device break down. It is desired to provide a semiconductorlight emitting device with high light emission efficiency whichsuppresses the degradation in device characteristics and the operationalmalfunction resulting from the generation of cracks or the introductionof defects due to the tensile stress included in the crystal layer orthe stress generated in high temperature operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating a semiconductorlight emitting device according to an embodiment;

FIG. 2A and FIG. 2B are schematic cross-sectional views showing thestress generated in stacked structures;

FIG. 3A and FIG. 3B are schematic cross-sectional views showing thestress generated in stacked structures including the support substrate;

FIG. 4A and FIG. 4B are graphs showing the Raman spectra of galliumnitride crystals;

FIG. 5 is a schematic cross-sectional view showing an example of thecrystal stacked structure when a semiconductor light emitting deviceaccording to the embodiment is fabricated;

FIG. 6A and FIG. 6B are schematic diagrams showing the stress generatedin another stacked structure;

FIG. 7 is a schematic cross-sectional view showing the stress generatedin a still another stacked structure;

FIG. 8A to FIG. 8C are schematic cross-sectional views showing processesfor fabricating the semiconductor light emitting device structure shownin FIG. 5;

FIG. 9A to FIG. 9D are schematic cross-sectional views showing processesfor fabricating the semiconductor light emitting device structure shownin FIG. 5; and

FIG. 10A to FIG. 10F are schematic cross-sectional views showingprocesses for fabricating the semiconductor light emitting devicestructure shown in FIG. 5.

DETAILED DESCRIPTION

In general, according to one embodiment, a semiconductor light emittingdevice includes: a first semiconductor layer of a first conductivitytype; a second semiconductor layer of a second conductivity type; alight emitting layer; a conductive metal layer; and a first stressapplication layer. The first semiconductor layer contains a nitridesemiconductor crystal and receives tensile stress in a (0001) plane. Thesecond semiconductor layer contains a nitride semiconductor crystal. Thelight emitting layer is provided between the first semiconductor layerand the second semiconductor layer and contains a nitride semiconductorcrystal. The light emitting layer has an average lattice constant largerthan a lattice constant of the first semiconductor layer. The conductivemetal layer is provided on an opposite side from the light emittinglayer, on the second semiconductor layer. The conductive metal layer hasa thermal expansion coefficient larger than a thermal expansioncoefficient of a nitride semiconductor crystal. The conductive metallayer supports the first semiconductor layer, the light emitting layer,and the second semiconductor layer. The first stress application layeris provided between the second semiconductor layer and the lightemitting layer. The first stress application layer relaxes tensilestress applied from the metal layer to the second semiconductor layer.

Hereinbelow, embodiments of the invention are described with referenceto the drawings.

The drawings are schematic or conceptual; and the relationships betweenthe thickness and width of portions, the proportions of sizes amongportions, etc. are not necessarily the same as the actual valuesthereof. Further, the dimensions and proportions may be illustrateddifferently among drawings, even for identical portions.

In the specification of this application and the drawings, componentssimilar to those described in regard to a drawing thereinabove aremarked with the same reference numerals, and a detailed description isomitted as appropriate.

FIG. 1 is a schematic cross-sectional view illustrating theconfiguration of a semiconductor light emitting device according to anembodiment.

As shown in FIG. 1, a semiconductor light emitting device 110 accordingto the embodiment includes a first semiconductor layer 10 of a firstconductivity type, a second semiconductor layer 20 of a secondconductivity type, a light emitting layer 30, a first stress applicationlayer 22, and a support substrate (a metal layer) 40. The semiconductorlight emitting device 110 is, for example, an LED device. Thesemiconductor light emitting device 110 may be also a laser diode. Inthe following, a description is given using the case where thesemiconductor light emitting device 110 is an LED.

An n-type semiconductor layer, for example, is used as the firstsemiconductor layer 10. A p-type semiconductor layer, for example, isused as the second semiconductor layer 20. However, the firstsemiconductor layer 10 may be the p type and the second semiconductorlayer 20 may be the n type. In the following, a description is givenusing the case where the first semiconductor layer 10 is the n type andthe second semiconductor layer 20 is the p type.

The first semiconductor layer 10 and the second semiconductor layer 20contain a nitride semiconductor crystal. As described later, the firstsemiconductor layer 10 inherently has tensile strain (elastic expansionand contraction of the lattice spacing resulting from receiving stress)in the in-plane direction due to the stress applied from the outside(e.g. the static force applied to the crystal). That is, due to tensilestress being applied to the first semiconductor layer 10 in the in-planedirection, the lattice length (the lattice spacing in the actual crystallattice) in the in-plane direction of the first semiconductor layer 10has been made longer than the original lattice constant (the valuedetermined as a physical constant) of the first semiconductor layer 10.

The first semiconductor layer 10 is, for example, an n-type GaN layer.The second semiconductor layer 20 is, for example, a p-type GaN layer.The first semiconductor layer 10 may include, for example, an i-GaNlayer (hereinafter, also referred to as a “non-doped GaN layer”) and ann-type GaN layer. The n-type GaN layer is laminated between the i-GaNlayer and the second semiconductor layer 20.

The light emitting layer 30 is provided between the first semiconductorlayer 10 and the second semiconductor layer 20. The light emitting layer30 contains a nitride semiconductor crystal. The light emitting layer 30contains a nitride semiconductor crystal having a lattice constant stilllarger than the lattice length in the in-plane direction of the firstsemiconductor layer 10 that has been expanded in the in-plane directionby receiving tensile stress.

The light emitting layer 30 includes, for example, a plurality ofbarrier layers 34 and a well layer 32 provided between barrier layers34. The well layer 32 may be provided in plural. For example, the lightemitting layer 30 has an MQW (multiple quantum well) structure.

The well layer 32 and the barrier layer 34 contain a nitridesemiconductor crystal.

The well layer 32 contains a nitride semiconductor crystal having alattice constant still larger than the lattice length in the in-planedirection of the first semiconductor layer 10 that has been expanded inthe in-plane direction by receiving tensile stress.

The lattice in the in-plane direction of the well layer 32 includescompressive strain (elastic expansion and contraction of the latticespacing) and has a lattice length smaller than the original latticeconstant in the in-plane direction of the well layer 32, as a result ofreceiving compressive stress from the lattice of the first semiconductorlayer 10 having a lattice length smaller than the lattice constant ofthe well layer 32.

The average lattice constant in the light emitting layer 30 is thelattice constant obtained by weighting by thickness distribution andaveraging the lattice constant of the barrier layer 34 and the latticeconstant of the well layer 32. The average lattice constant of the lightemitting layer 30 is larger than the lattice constant of the firstsemiconductor layer 10. Furthermore, the average lattice constant of thelight emitting layer 30 is larger than the lattice length in thein-plane direction of the first semiconductor layer 10 that has beenexpanded in the in-plane direction by receiving tensile stress.

The average lattice constant in the light emitting layer 30 may becomelarger from the first semiconductor layer 10 toward the secondsemiconductor layer 20 in the light emitting layer 30. Alternatively,the thicknesses of the plurality of well layers 32 in the light emittinglayer 30 having an MQW structure may become thicker from the firstsemiconductor layer 10 toward the second semiconductor layer 20.

The first stress application layer 22 is provided between the secondsemiconductor layer 20 and the light emitting layer 30. The first stressapplication layer 22 contains a nitride semiconductor crystal. Thelattice constant of the first stress application layer 22 is smallerthan the lattice constant of the first semiconductor layer 10. The firststress application layer 22 relaxes the tensile stress applied from thesupport substrate 40.

The support substrate 40 is provided on one surface of the secondsemiconductor layer 20, on the opposite side to the light emitting layer30. The support substrate 40 is a conductive metal layer. A metal suchas copper, for example, is used for the support substrate 40. At leastone of a reflection metal 90 and a bonding metal may be interposedbetween the support substrate 40 and the second semiconductor layer 20.In the semiconductor light emitting device 110 shown in FIG. 1, thereflection metal 90 is interposed between the support substrate 40 andthe second semiconductor layer 20.

The semiconductor light emitting device 110 may further include a secondstress application layer 16 (see FIG. 5). The second stress applicationlayer 16 is provided on the opposite side to the light emitting layer 30from the first semiconductor layer 10. For example, the second stressapplication layer 16 contact with the first semiconductor layer 10. Thesecond stress application layer 16 applies compressive stress to thefirst semiconductor layer 10, and thereby relaxes the tensile stressapplied from the support substrate 40. The stress is described later.

The semiconductor light emitting device 110 further includes a firstelectrode 81, a second electrode 82, and the reflection metal 90. Thesupport substrate 40 containing a metal such as copper is bonded to theLED stacked structure including the second semiconductor layer 20, thelight emitting layer 30, and the first semiconductor layer 10 via thereflection metal 90 containing Ag on the second semiconductor layer 20side and a bonding metal containing AuSn.

As illustrated by arrow 30L shown in FIG. 1, the light emitted from thelight emitting layer 30 is emitted from the major surface (lightextraction surface) on the first semiconductor layer 10 side. In otherwords, the light emitted from the light emitting layer 30 is emitted tothe outside of the semiconductor light emitting device 110 via the firstsemiconductor layer 10. Thus, the major surface on the firstsemiconductor layer 10 side forms a light extraction surface. The lightextraction surface may have undergone roughening processing. The LEDstacked structure of nitride semiconductor crystals is formed betweenthe second semiconductor layer 20 and the light extraction surface.

The first semiconductor layer 10 is made of, for example, an n-typegallium nitride (GaN) crystal. The light emitting layer 30 formed of amultiple-layer film of the well layer 32 and the barrier layer 34 isstacked on the first semiconductor layer 10. InGaN, for example, is usedfor the well layer 32. GaN, for example, is used for the barrier layer34. The second semiconductor layer 20 is stacked on the quantum wellstructure of nitride semiconductors that forms the light emitting layer30. The second semiconductor layer 20 is made of, for example, a p-typegallium nitride crystal.

In the specification of this application, “stack” includes not only thestate where a plurality of layers are stacked in contact with oneanother but also the case where a plurality of layers are stacked viaother layers.

In the specification of this application, being provided “on” includesnot only the case where an upper layer is provided in contact with alower layer but also the case where an upper layer is provided on alower layer via another layer.

The lattice constant of the well layer 32 (e.g. an InGaN crystal layer)included in the light emitting layer 30 is larger than the latticeconstant of the first semiconductor layer 10 (e.g. gallium nitride). Thesemiconductor light emitting device 110 has a structure in which a firstnitride semiconductor crystal (e.g. the GaN crystal that forms the firstsemiconductor layer 10 and the second semiconductor layer 20) is used asmatrices and a second nitride semiconductor crystal (the InGaN layerthat forms the well layer 32) having a lattice constant larger than thelattice constant of the first nitride semiconductor crystal is placedbetween the matrices.

In the case where all of the first semiconductor layer 10, the lightemitting layer 30, and the second semiconductor layer 20 are formed of ahexagonal crystal and are stacked in the c-axis direction, the a-axislength (the lattice length in the a-axis direction) of the lattice ofthe first semiconductor crystal layer is longer than the intrinsica-axis length (the lattice constant in the a-axis direction) of the GaNcrystal. That is, the first semiconductor crystal layer has tensilestress (elastic expansion and contraction of the lattice spacing) in thea-axis direction, as a result of the inherent tensile stress from theoutside being applied. The average lattice constant in the a-axisdirection of the lattice of the light emitting layer 30 (e.g. thestacked body of the well layer 32 of InGaN and the barrier layer 34 ofGaN) is longer than the a-axis length of the lattice of the firstsemiconductor crystal. The average lattice constant in the a-axisdirection of the lattice in the light emitting layer 30 may becomelarger from the first semiconductor layer 10 toward the secondsemiconductor layer 20 in the light emitting layer 30. The thicknessesof the plurality of well layers 32 in the light emitting layer 30 maybecome thicker from the first semiconductor layer 10 toward the secondsemiconductor layer 20. The magnitude of the tensile stress applied tothe GaN layer can be investigated by Raman spectroscopy as describedlater.

The support substrate 40 is provided on the opposite side to the lightemitting layer 30, on the second semiconductor layer 20. The supportsubstrate 40 may be formed of copper or a metal containing copper. Thethermal conductivity of the support substrate 40 is higher than thethermal conductivity of the nitride semiconductor crystal including thefirst semiconductor layer 10, the second semiconductor layer 20, and thelight emitting layer 30. The thermal expansion coefficient of thesupport substrate 40 is larger than the thermal expansion coefficient ofthe nitride semiconductor crystal including the first semiconductorlayer 10, the second semiconductor layer 20, and the light emittinglayer 30.

FIG. 2A and FIG. 2B are schematic cross-sectional views showing thestress generated in stacked structures. FIG. 3A and FIG. 3B areschematic cross-sectional views showing the stress generated in stackedstructures including the support substrate.

FIG. 4A and FIG. 4B are graphs showing the Raman spectra of galliumnitride crystals.

FIG. 2A and FIG. 3A illustrate the stress generated in the LED stackedstructure of the semiconductor light emitting device according to theembodiment. FIG. 2B and FIG. 3B illustrate the stress generated in theLED stacked structure of a semiconductor light emitting device of areference example. FIG. 4A is a graph showing the Raman spectrum of agallium nitride crystal and a silicon crystal. FIG. 4B is a graphshowing a comparison between the Raman spectrum of a nitridesemiconductor crystal grown on a silicon substrate and the Ramanspectrum of a nitride semiconductor crystal grown on a sapphiresubstrate. The vertical axis of FIG. 4A and FIG. 4B represents theintensity I (an arbitrary unit). The horizontal axis of FIG. 4A and FIG.4B represents the wave number RS (cm⁻¹).

As shown in FIG. 2B, in a semiconductor light emitting device 119 aaccording to the reference example, a gallium nitride crystal layer witha surface of the (0001) plane is formed on a sapphire substrate 55having a surface of the (0001) plane, and further the light emittinglayer 30 including an InGaN thin-film crystal layer is combined. Eachsemiconductor crystal of the semiconductor light emitting device 119 aof the reference example formed on the sapphire substrate 55 with asurface of the (0001) plane is oriented in the c-axis direction.

A light emitting diode in which nitride semiconductor crystal layers arestacked on the sapphire substrate 55 like the semiconductor lightemitting device 119 a has a structure in which an n-type GaN layer (thefirst semiconductor layer 10), a quantum well light emitting layer (thelight emitting layer 30), and a p-type GaN layer (the secondsemiconductor layer 20) are stacked on the sapphire substrate 55. Thesapphire substrate 55 is almost transparent to the wavelength band ofthe blue color region. Thus, for example, a structure is employed inwhich a reflection film 57 is formed on the back surface of the sapphiresubstrate 55 and light is thereby extracted from the upper portion ofthe p-type GaN layer on the front surface side (the face-up structure).

In the LED using gallium nitride epitaxially grown on the sapphiresubstrate 55, the equivalent lattice length of the sapphire crystalserving for the lattice for the epitaxial growth of gallium nitride issmaller than the lattice constant of gallium nitride. The thermalexpansion coefficient of the gallium nitride crystal is smaller than thethermal expansion coefficient of the sapphire crystal serving as theunderlayer. Therefore, as illustrated by arrow A1 and arrow A2 shown inFIG. 2B, a large compressive stress is applied to the gallium nitridecrystal layer when thin-film crystal growth at high temperature has beencompleted and the temperature has been lowered to room temperature. Thegallium nitride crystal layer has compressive strain (elastic expansionand contraction of the lattice spacing). That is, the lattice length inthe a-axis direction of the gallium nitride crystal layer epitaxiallygrown on the sapphire substrate 55 is shorter than the original latticeconstant in the a-axis direction of the gallium nitride crystal.

The lattice constant of the InGaN crystal layer included in the lightemitting layer 30 is larger than the lattice constant of galliumnitride. Therefore, as illustrated by arrow A3 and arrow A4 shown inFIG. 2B, stress in the drawing direction (tensile stress) is appliedfrom the InGaN crystal layer to the gallium nitride crystal layer towhich the compressive stress from the sapphire crystal has been applied.On the other hand, as illustrated by arrow A5 and arrow A6 shown in FIG.2B, the light emitting layer 30 receives compressive stress from thegallium nitride crystal layer. Such compressive stress and tensilestress are, in other words, generated in the a-axis direction in the(0001) plane, for example.

Thus, the tensile stress applied to the gallium nitride crystal layerfrom the InGaN crystal layer having a lattice constant larger than thelattice constant of gallium nitride is relatively balanced with thecompressive stress applied to the gallium nitride layer from thesapphire crystal. Therefore, there are few cases where defects aregenerated from the end surface of the n-type GaN layer, or the endsurface of the p-type GaN layer, etc.

On the other hand, under operating conditions where current injection isincreased for the purpose of higher light output, measures against heatgeneration are taken. To this end, for example as shown in FIG. 3B, astructure is employed in which an LED structure made of nitridesemiconductors is epitaxially grown on the sapphire substrate 55, thenthe surface side of the p-type GaN layer is attached to the supportsubstrate 40 with a high thermal conductivity, and the sapphiresubstrate 55 is peeled off (the thin-film structure).

In the thin-film structure, to promote the heat radiation in operationat high current density (high temperature operation), a metal such ascopper may be used as the support substrate 40. In this case, thethermal expansion coefficient of a metal such as copper is generallylarger than the thermal expansion coefficient of the nitridesemiconductor crystal including the first semiconductor layer 10, thesecond semiconductor layer 20, and the light emitting layer 30.

Specifically, the thermal expansion coefficient of gallium nitride is5.6×10⁻⁶ K⁻¹. In contrast, the thermal expansion coefficient of copperis 16.8×10⁻⁶ K⁻¹. That is, when the LED device of the thin-filmstructure using copper as the support substrate 40 is operated at hightemperature, the support substrate 40 expands more than the nitridesemiconductor crystal layer, as illustrated by arrow A7 and arrow A8shown in FIG. 3B. Consequently, the nitride semiconductor crystal layerreceives tensile stress from the support substrate 40.

Here, the findings by the inventors have revealed that the compressivestrain due to the compressive stress applied to the gallium nitridecrystal layer from the sapphire substrate 55 remains also in thethin-film structure from which the sapphire substrate 55 has beenremoved. That is, even after the sapphire substrate 55 is removed, thelattice length in the a-axis direction of the gallium nitride crystallayer is shorter than the original lattice constant in the a-axisdirection of the gallium nitride crystal. That is, in the thin-filmstructure in which a crystal of an LED structure is grown on thesapphire substrate 55 and then the sapphire substrate 55 is removed, thecompressive strain (elastic expansion and contraction of the latticespacing resulting from receiving stress) that has remained in theinterior of crystal is balanced also when the tensile stress from thesupport substrate 40 is applied in high temperature operation.Consequently, withstanding properties against the crack formation by thestress are high.

On the other hand, to utilize a substrate with a relatively large area,less expensive and more efficient in manufacturing processes than asapphire substrate, it is attempted to grow a gallium nitride crystal ona silicon crystal.

As shown in FIG. 2A, the semiconductor light emitting device 110according to the embodiment has an LED stacked structure that is formedon a silicon crystal with a surface of the (111) plane and includes ann-type GaN layer (the first semiconductor layer 10), a quantum welllight emitting layer (the light emitting layer 30), and a p-type GaNlayer (the second semiconductor layer 20) stacked. Each semiconductorcrystal of the semiconductor light emitting device 110 formed on asilicon substrate 50 with a surface of the (111) plane is oriented inthe c-axis direction.

The equivalent lattice length of the silicon crystal with a surface ofthe (111) plane for the lattice of epitaxial growth of gallium nitrideis larger than the lattice constant in the a-axis direction of galliumnitride. The thermal expansion coefficient of the silicon crystal issmaller than the thermal expansion coefficient of gallium nitride.Therefore, as illustrated by arrow A11 and arrow A12 shown in FIG. 2A, astrong tensile stress is applied to the gallium nitride crystal layerafter crystal growth is finished. The gallium nitride crystal layer hastensile strain (elastic expansion and contraction of the latticespacing). Furthermore, as illustrated by arrow A13 and arrow A14 shownin FIG. 2A, the nitride semiconductor crystal system formed on thesilicon crystal receives further tensile stress from the InGaN crystallayer. On the other hand, as illustrated by arrow A15 and arrow A16shown in FIG. 2A, the light emitting layer 30 receives compressivestress from the gallium nitride crystal layer. Such compressive stressand tensile stress are, in other words, generated in the a-axisdirection in the (0001) plane, for example.

Thus, in the semiconductor light emitting device 110 according to theembodiment, the tensile stress applied to the gallium nitride crystallayer from the InGaN crystal layer having a lattice constant larger thanthe lattice constant of gallium nitride synergizes with the tensilestress applied to the gallium nitride crystal layer from the siliconcrystal. Therefore, in the case where a nitride semiconductor crystal isepitaxially grown on the silicon substrate 50 to form a semiconductorlight emitting device, the tensile stress accumulated in the epitaxialcrystal layer may not only cause elastic deformation (strain) of thecrystal lattice but also generate cracks, defects, etc. as plasticdeformation of the crystal. Consequently, obstruction may be caused indevice fabrication processes and device operation, or the devicecharacteristics may be degraded.

In the case where a nitride crystal grown on the silicon substrate 50 isused as a semiconductor light emitting device, the silicon substrate isgenerally not transparent to the wavelength of the light used. Hence,the thin-film structure is employed in which the grown layer is peeledoff from the silicon substrate.

As described above, to promote the heat radiation during operation athigh current density (high temperature operation), a metal such ascopper and aluminum may be used as the support substrate 40. In thiscase, as described above, the thermal expansion coefficient of a metalhaving a high thermal conductivity, such as copper and aluminum, isgenerally larger than the thermal expansion coefficient of the nitridesemiconductor crystal. When the LED device is operated at hightemperature, the support substrate 40 expands more than the nitridesemiconductor crystal layer, as illustrated by arrow A17 and arrow A18shown in FIG. 3A. Consequently, the nitride semiconductor crystal layerreceives tensile stress from the support substrate 40.

As described above, in the thin-film structure in which the growthsubstrate is peeled off from the gallium nitride crystal of the LEDstructure, the tensile stress (elastic expansion and contraction of thelattice spacing) generated by the stress that is accumulated in thenitride semiconductor crystal layer after the growth due to thedifference between the lattice constant of the substrate and the latticeconstant of the crystal layer or the difference between the thermalexpansion coefficient of the substrate and the thermal expansioncoefficient of the crystal layer remains even after the growth substrateis peeled off. In high temperature operation, the nitride semiconductorcrystal layer receives not only the tensile stress that has remained inthe interior of crystal, but also the tensile stress resulting from thedifference between the thermal expansion coefficient of the supportsubstrate 40 and the thermal expansion coefficient of the crystal layer.That is, in the case where an LED structure made of nitridesemiconductor crystals is grown on the Si substrate 50, tensile strain(elastic expansion and contraction of the lattice spacing) is includedin the GaN crystal layer. The lattice length in the a-axis direction ofthe GaN crystal is larger than the original lattice constant in thea-axis direction of the GaN crystal. Therefore, in the case where thesupport substrate 40 formed of a metal such as copper is used in thethin-film structure, in high temperature operation, the tensile stressresulting from the difference between the thermal expansion coefficientof the support substrate 40 and the thermal expansion coefficient of thecrystal layer works synergistically with the tensile strain included inthe crystal layer. Thereby, the risk of plastic deformation such asdefects and cracks will be caused is increased in high temperatureoperation, and it results in characteristic degradation and deviceoperational malfunction.

When the In composition ratio of the light emitting layer 30 is high andthe average lattice length of the light emitting layer 30 is large, thetensile stress applied from the InGaN crystal layer to the galliumnitride crystal layer is large, and malfunction occurring in operationis significant. Also when the thickness of the InGaN crystal layer isthick, malfunction occurring in operation is significant.

In the case where the average lattice constant in the light emittinglayer 30 becomes larger from the first semiconductor layer 10 toward thesecond semiconductor layer 20, a larger tensile stress is applied to thesecond semiconductor layer 20. Therefore, in the case where tensilestress is applied from the support substrate 40 to the secondsemiconductor layer 20, the risk of the formation of crystal defects andcracks is further increased. Specifically, in the case where thethicknesses of the plurality of InGaN well layers 32 in the lightemitting layer 30 become thicker from the first semiconductor layer 10toward the second semiconductor layer 20, a larger tensile stress iseffectively applied to the second semiconductor layer 20. Thus, thetensile stress applied from the support substrate 40 works moresynergistically.

In contrast, in the semiconductor light emitting device 110 according tothe embodiment, as shown in FIG. 1 and FIG. 3A, the first stressapplication layer 22 is provided between the second semiconductor layer20 and the light emitting layer 30. Thereby, the first stressapplication layer 22 relaxes the tensile stress applied from the supportsubstrate 40.

The first stress application layer 22 includes, for example, an AlGaNlayer. The first stress application layer 22 is not limited to includingone AlGaN layer but may include a plurality of AlGaN layers.

By the semiconductor light emitting device 110 according to theembodiment, the first stress application layer 22 can relax the tensilestress applied from the support substrate 40 even under conditions wheretensile strain remains in the first semiconductor crystal. Therefore,the formation of cracks or the introduction of defects in hightemperature operation can be suppressed, and a semiconductor lightemitting device with high light emission efficiency can be provided.Specifically, in the case where the support substrate 40 formed of ametal such as copper is used in the thin-film structure, even whentensile stress is generated in operation at high current density (inhigh temperature operation), the formation of cracks or the introductionof defects can be suppressed, and a semiconductor light emitting devicewith high light emission efficiency can be provided.

For example, in an device structure in which, as described in FIG. 2A,the support substrate 40 that applies tensile stress is further includedin the thin-film crystal having tensile stress (the first semiconductorlayer 10), the first stress application layer 22 for relaxing thetensile stress applied from the support substrate 40 is laminated.Therefore, the degradation in device characteristics resulting from theintroduction of defects due to tensile stress can be suppressed.

Whether the stress applied to the gallium nitride crystal layer iscompressive stress or tensile stress can be determined from a Ramanspectrum. For example, the peak of the Raman spectrum of a galliumnitride crystal to which no stress is applied is at approximately 568cm⁻¹, whereas the peak appears at a wave number RS smaller than 568cm⁻¹, for example approximately 567.8 to 566 cm⁻¹, in a gallium nitridecrystal to which tensile stress is applied, and the peak appears at awave number RS larger than 568 cm⁻¹, up to approximately 575 cm⁻¹, in agallium nitride crystal to which compressive stress is applied.According to the graph shown in FIG. 4B, it is found that a tensilestress of 0.1 to 0.15% is included in a GaN crystal layer grown on thesilicon substrate 50.

FIG. 5 is a schematic cross-sectional view showing an example of thecrystal stacked structure when a semiconductor light emitting deviceaccording to the embodiment is fabricated.

As shown in FIG. 5, in a semiconductor light emitting device 120according to the embodiment, a buffer layer 12 (a layer that can form asecond stress application layer described later) including an AlN layerand an AlGaN layer is laminated on the silicon substrate 50. An AlNlayer (which plays a stress controlling layer during growth, and is tobe the second stress application layer in the device.) 16 with athickness of 15 nanometers (nm) is provided on the buffer layer 12 via anon-doped GaN layer 14 with a thickness of 300 nm. The firstsemiconductor layer 10 is stacked on the AlN layer 16. An n-type GaNlayer 18 with a thickness of 2 micrometers (μm) and a non-doped GaNlayer 17 with a thickness of 1 μm are stacked in the first semiconductorlayer 10.

An SLS (super lattice structure) layer 60 having a structure in which aGaN layer with a thickness of 3 nm and an InGaN layer with an In contentof 7% and a thickness of 1 nm are formed repeatedly 30 times islaminated on the n-type Gan layer 18. The MQW light emitting layer 30 isstacked on the SLS layer 60. The MQW light emitting layer 30 has astructure in which the barrier layer 34 with a thickness of 5 nm made ofGaN and the well layer 32 formed of an InGaN layer with an In content of15% and a thickness of 3 nm are formed repeatedly 8 times. In thesemiconductor light emitting device 120 of the embodiment, thecomposition ratio of In in the well layer 32 is, for example, not lessthan 0.12 and not more than 0.20.

A p-type AlGaN layer (the first stress application layer 22) with an Alcontent of 20% is laminated on the light emitting layer 30. A p-type GaNlayer (the second semiconductor layer 20) is laminated on the p-typeAlGan layer (the first stress application layer 22). The reflectionmetal 90 is laminated on the p-type GaN layer (the second semiconductorlayer 20).

The support substrate 40 of copper is attached onto the reflection metal90 via a bonding metal. After the support substrate 40 is attached inaccordance with the processes described later and an n electrode isformed, the silicon substrate 50 for epitaxial growth is peeled off;thus, the process is completed. Herein, differing from the semiconductorlight emitting device 110 shown in FIG. 1, the second stress applicationlayer 16 is interposed between the buffer layer 12 and the firstsemiconductor layer 10 (an n-type GaN layer). The second stressapplication layer 16 can be taken into the device by not removing butleaving the stress controlling layer during growth.

Here, as described above, the thermal expansion coefficient of galliumnitride is 5.6×10⁻⁶ K⁻¹. In contrast, the thermal expansion coefficientof copper is 16.8×10⁻⁶ K⁻¹. For the operation of a high power LEDdevice, usually a current of 350 mA or more is passed. At this time, thetemperature in the device may reach approximately 100° C. to 200° C.When the operating temperature is 200° C., the thermal expansion rate ofgallium nitride is approximately 0.11%. In contrast, the thermalexpansion rate of copper, which is the support substrate 40, isapproximately 0.33%. The thermal expansion rate of copper, which is thesupport substrate 40, is approximately three times the thermal expansionrate of gallium nitride. That is, a tensile stress equivalent to thedifference of 0.22% is applied to the gallium nitride crystal layer.

On the other hand, the lattice constant in the in-plane direction (thea-axis direction) of a GaN crystal is 0.518 nm. The lattice constant inthe in-plane direction (the a-axis direction) of an AlN crystal is 0.498nm. The difference between the lattice constant in the in-planedirection (the a-axis direction) of the GaN crystal and the latticeconstant in the in-plane direction (the a-axis direction) of the AlNcrystal is approximately 4%. Therefore, an AlGaN crystal with an Alcontent of approximately 5% (the lattice constant in the a-axisdirection being approximately 0.517 nm, the mismatch factor to the GaNcrystal being 0.2%) is suitable as the first stress application layer22. However, considering that tensile strain may remain in the GaNcrystal layer at a maximum of approximately 0.5 to 1.0%, the Alcomposition ratio of the AlGaN layer that forms the first stressapplication layer 22 may be as high as approximately 30%. The thicknessof the first stress application layer 22 is set within a range of 5 to20 nm.

With regard to the second stress application layer 16, since it is awayfrom the bonding interface between the support substrate 40 of copperincluding the reflection metal and the p-type GaN layer 20 via the lightemitting layer 30, an AlGaN layer with an Al content higher than the Alcontent of the first stress application layer 22 is suitable. That is,the lattice constant of the second stress application layer 16 ispreferably smaller than the lattice constant of the first stressapplication layer 22. Specifically, an AlN crystal layer is preferable.The thickness of the second stress application layer 16 is set to 10 to50 nm. The second stress application layer 16 may have a two-layerstructure of an AlN layer and an AlGaN layer with an Al content ofapproximately 50% or less.

FIG. 6A and FIG. 6B are schematic diagrams showing the stress generatedin another stacked structure.

FIG. 6A is a schematic cross-sectional view showing the configuration ofthe other stacked structure. FIG. 6B is a graph showing an example ofthe relationship between the position in the stacking direction and theinternal stress. In the specification of this application, the “stackingdirection” refers to the direction from the first semiconductor layer 10toward the second semiconductor layer 20 or the direction from thesecond semiconductor layer 20 toward the first semiconductor layer 10.

In a semiconductor light emitting device 130, the thicknesses of theplurality of well layers 32 in the light emitting layer 30 having an MQWstructure become thicker from the first semiconductor layer 10 towardthe second semiconductor layer 20. Specifically, for example, thethicknesses of the well layers 32 other than the well layer 32 nearestto the p-type GaN layer out of the plurality of well layers 32 is 3 nm.In contrast, the thickness of the well layer 32 nearest to the p-typeGaN layer is 5 nm. The thickness of the barrier layer 34 of the lightemitting layer 30 is 5 nm. This is similar to the thickness of thebarrier layer 34 of the semiconductor light emitting device 120described above in regard to FIG. 5.

As shown by arrow A23 and arrow A24 shown in FIG. 6A and FIG. 6B, in thestructure in which the width of the well layer 32 made of InGaN is wide,the tensile stress applied to the p-type GaN layer from the well layer32 is relatively strong. The tensile stress applied to the p-type GaNlayer from the well layer 32 is superposed with the tensile stressapplied to the p-type GaN layer from the support substrate 40 of copper.Thereby, the frequency of crack formation in high temperature operationis increased. Therefore, the effect of the first stress applicationlayer 22 made of an AlGaN crystal is more significant. In the casewhere, like this example, the average lattice constant becomes largereffectively from the n-type GaN layer (the first semiconductor layer 10)toward the p-type GaN layer (the second semiconductor layer 20) in thelight emitting layer 30, the effect of the first stress applicationlayer 22 is larger. However, in the case where the average latticeconstant becomes larger on the p-type GaN layer side, the differencebetween the lattice constant of the first stress application layer 22(an AlGaN layer) and the lattice constant of the well layer 32 is large,and the risk is increased that defects will occur at the interfacebetween the light emitting layer 30 and the first stress applicationlayer 22.

FIG. 7 is a schematic cross-sectional view showing the stress generatedin a still another stacked structure.

In a semiconductor light emitting device 140, similarly to thesemiconductor light emitting device 130 described above in regard toFIG. 6A and FIG. 6B, the thicknesses of the plurality of well layers 32in the light emitting layer 30 having an MQW structure become thickerfrom the first semiconductor layer 10 toward the second semiconductorlayer 20.

In this case, as shown in FIG. 7, making the interior of the firststress application layer 22 to be a two-layer structure is one effectivemeans. That is, an AlGaN layer with a relatively low Al content is usedas a first stress application unit 22 a on the side near to the lightemitting layer 30. An AlGaN layer with a relatively high Al content isused as a second stress application unit 22 b on the side near to thep-type GaN layer (the second semiconductor layer 20). Specifically, forexample, the first stress application unit 22 a on the side near to thelight emitting layer 30 is formed to be an AlGaN layer with an Alcontent of 10% and a thickness of 10 nm. The second stress applicationunit 22 b thereon on the side near to the p-type GaN layer is stacked tobe an AlGaN layer with an Al content of 20% and a thickness of 10 nm.Here, the structure of the first stress application layer 22 is notlimited to a two-layer structure of AlGaN layers. For example, the firststress application layer 22 may have a multiple-layer structure of threeor more layers, or may have a structure with a continuously gradientcomposition ratio. A structure in which the lattice constant becomessmaller from the light emitting layer 30 toward the p-type GaN layer ismore preferable.

Next, an example of the fabrication processes of the semiconductor lightemitting device 120 described above in regard to FIG. 5 is described.

FIG. 8A to FIG. 10F are schematic cross-sectional views showingprocesses for fabricating the semiconductor light emitting devicestructure shown in FIG. 5.

First, the silicon substrate 50 with a surface of the (111) plane isprepared as a substrate for the crystal growth of a thin-film nitridesemiconductor. The thickness of the crystal of the silicon substrate 50is, for example, approximately 525 μm. However, the thickness of thecrystal of the silicon substrate 50 is not limited thereto, and may be,for example, approximately 250 μm to 1000 μm.

In general, the surface of the Si substrate 50 placed in the air iscoated with a native oxide. To epitaxially grow a nitride semiconductorcrystal layer, it is necessary to remove the native oxide and reveal thesilicon crystal surface. To this end, hydrofluoric acid treatment isperformed on the silicon substrate 50 in order to perform hydrogentermination, which is a means to reveal the silicon crystal surface atrelatively low temperature. Specifically, after acid washing treatmentfor removing the contaminants on the substrate surface is performed, thesilicon substrate 50 is treated with a dilute hydrofluoric acid solutionwith a concentration of approximately 1% for about 1 minute. By thistreatment, the surface of the Si layer becomes a hydrogen-terminatedsurface structure, which is a water-repellent surface. The hydrogenatoms covering the silicon crystal surface desorbs at a temperature ofapproximately 700° C. Thereby, a clean silicon crystal surface can beobtained. On the other hand, as another means for obtaining a cleansilicon crystal surface, there is a method in which a silicon crystalsubstrate with the surface covered with a thin native oxide isheat-treated at a high temperature of 1000° C. or more.

Herein, the Si substrate 50 of which the surface has beenhydrogen-terminated is introduced into a film deposition apparatus (anMOCVD apparatus) using an organic metal and ammonia gas as the sourcematerial, and an AlN layer with a thickness of 100 nm is stacked at afilm deposition temperature of 1200° C. Although an example in which anMOCVD apparatus is used for the film deposition of the AlN layer isdescribed herein, the selection of the film deposition method isarbitrary. For example, also an ECR plasma sputtering apparatus, an MBEapparatus, etc. may be used as the film deposition apparatus of the AlNlayer.

In the case where the film deposition of the AlN layer on the Sisubstrate 50 is performed by other than an MOCVD apparatus, after thefilm deposition of the AlN layer, the substrate is introduced into anMOCVD apparatus, and subsequently the film deposition processes beloware performed.

After the AlN layer of 100 nm is stacked on the Si substrate 50, thesubstrate temperature is set to 1100° C. to stack an AlGaN layer with anAl content of 25% and a thickness of 250 nm.

The AlN layer and the AlGaN layer thus formed correspond to the bufferlayer 12 shown in FIG. 5.

After that, a gallium nitride layer of 0.3 to 1.0 μm is formed using TMG(trimethylgallium) and NH₃ (ammonia) as the source material. After thegallium nitride layer 14 of 0.3 to 1.0 μm is stacked, the filmdeposition temperature is lowered to 700° C. to grow an AlN layer 16with a thickness of 15 nm.

The AlN layer 16 functions as a stress controlling layer during growth.At this time, an AlGaN layer with a thickness of 50 nm and an Al contentof 25% may be further stacked on the AlN layer 16. The AlN layer 16 orthe stacked structure of the AlN layer 16 and the AlGaN layer may beused as the second stress application layer. Furthermore, the AlN layer16 with a thickness of 15 nm serving as a stress controlling layerduring growth may not be interposed, and an AlN layer that forms thebuffer layer 12 (alternatively, the buffer layer 12 formed of a stackedstructure of an AlN layer and an AlGaN layer) may be used as the secondstress application layer.

Subsequently, an n-type GaN (the first semiconductor layer) 10 isstacked. At this time, in the n-type GaN 10, Si is adoped as an impuritywith a concentration of 1×10¹⁹ cm⁻². Here, as shown in FIG. 5, then-type GaN 10 may not be formed directly on the AlN layer 16, but aintermediate layer (non-doped barrier layer) 17 containing no impuritymay be grown with a thickness of approximately 1 to 3 μm and then ann-type GaN layer 18 may be stacked. In other words, the firstsemiconductor layer 10 may have a structure in which the non-doped GaNlayer 17 and the n-type GaN layer 18 are stacked.

After the n-type GaN 10 is grown, the SLS layer 60 formed of amultiple-layer film of InGaN and GaN and the light emitting layer (MQWlight emitting layer) 30 are stacked on the n-type gallium nitridecrystal layer 10. To perform current injection to the light emittinglayer 30 to emit light, the upper portion of the crystal structure isdoped with a p-type impurity (Mg). At this time, an AlGaN layer (thefirst stress application layer 22) with an Al content of 20% and athickness of 150 nm and p-type GaN (the second semiconductor layer 20)not containing Al are formed on the light emitting layer 30.

Also the AlGaN layer may be doped with a p-type impurity (Mg). Thedoping concentration of Mg is preferably a concentration in a range of1×10¹⁹ to 1×10²⁰ cm⁻². However, in the case where a Mg doping layer isformed by the MOCVD method, the doping concentration changes due to thememory effect; therefore, a uniform doping profile is not necessarilyobtained. Thus, the doping concentration of Mg is allowed to deviatefrom the range described above.

Although herein the chemical vapor deposition method (the MOCVD method)using an organic metal is given as the method for the thin-film crystalgrowth of the n-type GaN crystal layer 10, the light emitting layer 30,the first stress application layer 22, and the p-type GaN 20, the methodis not limited thereto. As the method for the thin-film crystal growthof the n-type GaN crystal layer 10, the light emitting layer 30, thefirst stress application layer 22, and the p-type GaN 20, any methodsuch as the molecular beam epitaxy (MBE) method and the hydride vaporphase epitaxy (HVPE) method, which are thin-film crystal growth methodscommonly used for nitride semiconductor crystal growth, may be used.

Thus, as shown in FIG. 8A, a thin-film crystal layer (a crystal growthlayer) 70 of an LED structure can be epitaxially grown. After that, asshown in FIG. 8B, a metal film (the reflection metal 90) containing Agfunctioning as both a reflection film and a contact layer, for example asilver nickel layer, is stacked on the surface of the secondsemiconductor layer 20, and then the workpiece is attached to theconductive support substrate 40 of copper or the like via a bondingmetal (e.g. a gold-tin alloy (not shown)), with an intermediate layer(not shown) made of a metal material such as Ti, W, Pt, and Auinterposed therebetween. The thickness of the support substrate 40 ispreferably approximately 100 to 200 μm. An electrode is formed on thereflection metal 90 (or a bonding metal further stacked, or the supportsubstrate 40); thereby, a p-side electrode can be formed.

Next, the formation of an n-side electrode is described. As shown inFIG. 8C, the Si substrate 50, which is a substrate for thin-film crystalgrowth, is removed. The support substrate 40 is attached to the secondsemiconductor layer 20 side, and then the growth substrate is ground;thereby, the Si substrate 50 for growth can be removed. At this time,after the Si substrate 50 is almost removed by grinding, finally thesmall amount of residual Si is removed by dry etching using SF₆ gas asan etchant; thereby, the AlN layer (the buffer layer 12) formed on theSi substrate 50 at the beginning can be revealed.

Here, the AlN layer has the property of increasing the amount ofresistance components. Therefore, for example in a semiconductor lightemitting device having the stacked structure described above in regardto FIG. 5, there is an example in which the AlN-based buffer layer (e.g.the buffer layer 12 including an AlN layer) and the AlN-based stresscontrolling layer during growth are removed to reveal the n-type GaNlayer 18 and then an n-side electrode is provided.

Specifically, in terms of electrode formation, the AlN-based bufferlayer or the AlN layer has a high contact resistance. In addition, theamount of series resistance components is increased. Therefore, incommon processes, the AlN-based buffer layer and the AlN-based stresscontrolling layer during growth in the electrode formation portion areremoved to reveal the n-type GaN layer in the portion where the n-sideelectrode (the first electrode 81) will be formed.

On the other hand, in the following example, a description is givenincluding the case where, while the buffer layer 12 is removed, theAlN-based stress controlling layer during growth is utilized as thesecond stress application layer. As shown in FIG. 9A, the crystal growthlayer 70 is divided with the size of the device. At this time, thesubstrate side below the metal of a p-type electrode (the secondelectrode 82) is kept in the state of not being divided. Subsequently,as shown in FIG. 9B, portions other than the portion where an n-sideelectrode (the first electrode 81) will be formed are protected by amask 87, and the portion from the buffer layer 12 to the stresscontrolling layer during growth is removed by etching to reveal thefirst semiconductor layer 10 (n-type GaN). After that, as shown in FIG.9C, only the portion where the n-side electrode (the first electrode 81)will be formed is protected by a mask 89, and a KOH solution is used toperform roughening processing with a depth of approximately 500 nm onthe nitride semiconductor surface (the first semiconductor layer 10)side. At this time, the AlN and the AlGaN layer (the buffer layer 12)revealed at the surface are removed by etching. The AlN-based stresscontrolling layer during growth included in the crystal growth layer 70(the nitride semiconductor crystal layer unit 70 a) is removed by theroughening processing of the nitride semiconductor surface. However, theAlN-based stress controlling layer during growth may not be removed butbe used as the second stress application layer (see FIG. 9C).

Finally, as shown in FIG. 9D, a p-type electrode and an n-type electrodeare formed to complete the fabrication.

In the fabrication method described above, a sequence of forming thep-type electrode and the n-type electrode from both sides of the nitridesemiconductor crystal layer is described. However, it is also possibleto form both electrodes (the p-type electrode and the n-type electrode)on the opposite side to the light extraction surface. A specificfabrication method will now be described.

The process of epitaxial growth of a stacked structure of a nitridesemiconductor thin-film crystal having an LED structure on the siliconsubstrate 50 with a surface of the (111) plane is similar to the above,and a description is therefore omitted. After the epitaxial growth ofthe nitride semiconductor thin-film crystal growth layer of an LEDstructure including a p-type GaN crystal layer at the surface isfinished, first, a protection film 85 is formed on the entire surface ofthe p-type GaN layer. After that, an opening is partly provided, andetching processing is performed. By the etching processing, the p-typelayer, the first stress application layer 22, and the light emittinglayer 30 in the opening portion are etched to reveal the n-GaN layer 10(see FIG. 10A). After that, as shown in FIG. 10B, an ohmic contact 83made of Ti, Al, Ni, Au, or the like is formed on the revealed portion ofthe n-GaN layer.

After that, the protection film 85 covering the p-type GaN layer isremoved, and a protection film 84 is formed on the n electrode formationportion. Further, the reflection metal 90 made of silver or an alloycontaining silver as a main component is stacked on the surface of thep-type GaN layer. On the reflection metal 90, an intermediate layer (notshown) made of a metal such as, for example, Ti, W, Au, Pt, and Al isinterposed and further a bonding metal (not shown) made of a gold-tinalloy is stacked. After that, the support substrate 40 made of a metalsuch as copper is attached (see FIG. 10C). The thickness of the supportsubstrate is preferably approximately 100 to 200 μm.

As shown in FIG. 10D, after the support substrate 40 is attached, the Sisubstrate 50 used for the epitaxial growth is peeled off to reveal theAlN buffer layer 12. After the AlN buffer layer is revealed, rougheningprocessing is performed on the revealed surface. At this time, the AlNbuffer layer is removed. Further, the AlN-based stress controlling layerduring growth is removed. Alternatively, the AlN-based stresscontrolling layer during growth is not removed but used as the secondstress application layer 16 (see FIG. 10E). In such device fabricationprocesses, there is no n-side electrode on the light extraction surface.Therefore, roughening processing can be performed on the entire surface.

After the roughening processing is finished, the n electrode portion ofthe support substrate 40 of copper is opened. Subsequently, an nextension electrode, which is insulated from the support substrate 40,is revealed and an interconnection of the n electrode is connected. Thesubstrate 40 can be used as a p-side electrode (see FIG. 10F).

Here, as described above, in the case where the n electrode is formedfrom the n-GaN layer side, immediately after the growth of the thin-filmcrystal of an LED structure is finished, the support substrate 40 ofcopper is attached onto the flat p-GaN layer. In contrast, in theprocesses in which the n electrode is formed from the p-GaN layer side,the support substrate 40 of copper is attached onto the p-type GaN layerin which the n electrode portion is formed beforehand. That is, it isnecessary to attach the support substrate 40 of copper to a surfacepartly having the unevenness caused by processing (not flat). At thetime of the attachment, a bonding metal layer is interposed to followthe unevenness of the bonding surface. However, degradation in bondingconditions due to the formation of voids etc. is likely to occur ascompared to attachment to a flat surface. To address the problem, also amethod in which the support substrate 40 of copper is formed by aplating process, not attaching it as a plate-like substrate as describedabove, is one effective means.

Specifically, the preliminary processing of the formation of an nelectrode and a p electrode is performed on a substrate in which anitride semiconductor crystal layer of an LED structure is epitaxiallygrown on the silicon substrate 50. After that, the reflection metal 90made of silver or containing silver as a main component and anintermediate layer made of a metal such as Ti, W, and Pt are stacked bythe chemical vapor deposition method, the sputtering method, or thelike. Subsequently, a seed layer containing Ti and Cu is stacked, andfinally the plating method is used to form copper with a thickness ofapproximately 100 μm. When the support substrate 40 of copper is formedby the plating method, a structure with high adhesion to the unevennessof the electrode formation portion can be fabricated.

The material of the conductive support substrate 40 may be, in additionto copper (thermal conductivity: 370 to 380 Wm⁻¹K⁻¹, thermal expansioncoefficient: 16.6×10⁻⁶ K⁻¹), gold (thermal conductivity: 295 to 320Wm⁻¹K⁻¹, thermal expansion coefficient: 14.2×10⁻⁶ K⁻¹), silver (thermalconductivity: 418 Wm⁻¹K⁻¹, thermal expansion coefficient: 18.9×10⁻⁶K⁻¹), or the like. The material of the conductive support substrate 40may be also aluminum (thermal conductivity: 200 to 230 Wm⁻¹K⁻¹, thermalexpansion coefficient: 23.1×10⁻⁶ K⁻¹). The material of the conductivesupport substrate 40 may be also an alloy of two or more of the metalsmentioned above, an alloy using any of the metals mentioned above as amatrix, or the like. The conductive support substrate 40 may have astacked film structure in which a layer of any of the metals, includingalloys, mentioned above is used as a main layer and is combined withanother material. That is, the support substrate 40 of the embodimentcontains a metal selected from the group consisting of gold (Au), silver(Ag), copper (Cu), and aluminum (Al) or an alloy containing two or moreselected from the group mentioned above.

In the specification, “nitride semiconductor” includes allsemiconductors expressed by the chemical formula ofB_(x)In_(y)Al_(z)Ga_(1-x-y-z)N (0≦x≦1, 0≦y≦1, 0≦z≦1, x+y+z≦1) in whichthe composition ratios x, y, and z are changed in the respective ranges.Furthermore, in the chemical formula mentioned above, also those furthercontaining a group V device other than N (nitrogen), those furthercontaining various devices added in order to control various propertiessuch as the conductivity type, and those further containing variousdevices unintentionally contained are included in the “nitridesemiconductor.”

Hereinabove, embodiments of the invention are described with referenceto specific examples. However, the invention is not limited to thesespecific examples. For example, one skilled in the art may appropriatelyselect specific configurations of components of semiconductor lightemitting devices such as light emitting layers and semiconductor layersfrom known art and similarly practice the invention. Such practice isincluded in the scope of the invention to the extent that similareffects thereto are obtained.

Further, any two or more components of the specific examples may becombined within the extent of technical feasibility and are included inthe scope of the embodiments to the extent that the spirit of theembodiments is included.

Moreover, all semiconductor light emitting devices practicable by anappropriate design modification by one skilled in the art based on thesemiconductor light emitting devices described above as embodiments ofthe invention also are within the scope of the invention to the extentthat the purport of the embodiments of the invention is included.

Furthermore, various modifications and alterations within the spirit ofthe invention will be readily apparent to those skilled in the art.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A semiconductor light emitting device comprising:a first semiconductor layer of a first conductivity type containing anitride semiconductor crystal and receiving tensile stress in a (0001)plane; a second semiconductor layer of a second conductivity typecontaining a nitride semiconductor crystal; a light emitting layerprovided between the first semiconductor layer and the secondsemiconductor layer, containing a nitride semiconductor crystal, andhaving an average lattice constant larger than a lattice constant of thefirst semiconductor layer; a conductive metal layer provided on anopposite side to the light emitting layer, on the second semiconductorlayer, having a thermal expansion coefficient larger than a thermalexpansion coefficient of a nitride semiconductor crystal, and supportingthe first semiconductor layer, the light emitting layer, and the secondsemiconductor layer; and a first stress application layer providedbetween the second semiconductor layer and the light emitting layer andrelaxing tensile stress applied from the metal layer to the secondsemiconductor layer.
 2. The device according to claim 1, wherein anaverage lattice constant of the light emitting layer becomes larger fromthe first semiconductor layer toward the second semiconductor layer. 3.The device according to claim 1, wherein the light emitting layerincludes: a plurality of barrier layers; and a plurality of well layerseach provided between adjacent ones of the plurality of barrier layersand thicknesses of the plurality of well layers become larger from thefirst semiconductor layer toward the second semiconductor layer.
 4. Thedevice according to claim 1, wherein the first stress application layerincludes a plurality of layers with different lattice constants andlattice constants of the plurality of layers become larger from thesecond semiconductor layer toward the light emitting layer.
 5. Thedevice according to claim 1, further comprising a second stressapplication layer provided on an opposite side to the light emittinglayer, on the first semiconductor layer and applying compressive stressto the first semiconductor layer.
 6. The device according to claim 5,wherein a lattice constant of the second stress application layer issmaller than a lattice constant of the first stress application layer.7. The device according to claim 1, wherein the first semiconductorlayer contains GaN.
 8. The device according to claim 1, wherein thesecond semiconductor layer contains GaN.
 9. The device according toclaim 1, wherein the light emitting layer contains InGaN.
 10. The deviceaccording to claim 1, wherein the first stress application layercontains AlGaN.
 11. The device according to claim 1, wherein the metallayer contains a metal selected from the group consisting of gold (Au),silver (Ag), copper (Cu), and aluminum (Al) or an alloy containing twoor more selected from the group.
 12. The device according to claim 5,wherein the second stress application layer contains AlN.
 13. The deviceaccording to claim 12, wherein the second stress application layercontains AlGaN.
 14. A semiconductor light emitting device comprising: afirst semiconductor layer of a first conductivity type containing anitride semiconductor crystal and having tensile strain in a (0001)plane; a second semiconductor layer of a second conductivity typecontaining a nitride semiconductor crystal; a light emitting layerprovided between the first semiconductor layer and the secondsemiconductor layer, containing a nitride semiconductor crystal, andhaving an average lattice constant larger than a lattice constant of thefirst semiconductor layer; a conductive metal layer provided on anopposite side to the light emitting layer, on the second semiconductorlayer, having a thermal expansion coefficient larger than a thermalexpansion coefficient of a nitride semiconductor crystal, and supportingthe first semiconductor layer, the light emitting layer, and the secondsemiconductor layer; and a first stress application layer providedbetween the second semiconductor layer and the light emitting layer,containing a nitride semiconductor crystal, and having a latticeconstant smaller than a lattice constant of the first semiconductorlayer.
 15. The device according to claim 14, wherein an average latticeconstant of the light emitting layer becomes larger from the firstsemiconductor layer toward the second semiconductor layer.
 16. Thedevice according to claim 14, wherein the light emitting layer includes:a plurality of barrier layers; and a plurality of well layers eachprovided between adjacent ones of the plurality of barrier layers andthicknesses of the plurality of well layers become larger from the firstsemiconductor layer toward the second semiconductor layer.
 17. Thedevice according to claim 14, wherein the first stress application layerincludes a plurality of layers with different lattice constants andlattice constants of the plurality of layers become larger from thesecond semiconductor layer toward the light emitting layer.
 18. Thedevice according to claim 14, further comprising a second stressapplication layer provided on an opposite side to the light emittinglayer, on the first semiconductor layer and applying compressive stressto the first semiconductor layer.
 19. The device according to claim 18,wherein a lattice constant of the second stress application layer issmaller than a lattice constant of the first stress application layer.20. The device according to claim 14, wherein the first semiconductorlayer contains GaN.
 21. The device according to claim 14, wherein thesecond semiconductor layer contains GaN.
 22. The device according toclaim 14, wherein the light emitting layer contains InGaN.
 23. Thedevice according to claim 14, wherein the first stress application layercontains AlGaN.
 24. The device according to claim 14, wherein the metallayer contains a metal selected from the group consisting of gold (Au),silver (Ag), copper (Cu), and aluminum (Al) or an alloy containing twoor more selected from the group.
 25. The device according to claim 18,wherein the second stress application layer contains AlN.
 26. The deviceaccording to claim 25, wherein the second stress application layercontains AlGaN.